The proposed work is a logical extension of the candidates extensive clinical background in the field of emergency cardiac care; now focused on the cellular mechanisms of ischemia and reperfusion. Dr. Becker is board certified in Critical Care, Internal, and Emergency Medicine, and has completed graduate studies in biochemistry. He has an appointment as Assistant Professor and Research Director in the Section Of Emergency Medicine, Department Of Medicine at the University of Chicago. Based on preliminary data using a perfused cardiomyocyte model, many cells which are viable at the end of ischemia subsequently die during the first minutes of reperfusion. Metal chelation significantly reduces cell death, suggesting a role for metal-catalyzed free radical generation. This fatal injury occurs in an isolated cell system without neutrophils, inflammatory mediators, endothelium, xanthine oxidase, or catecholamines. This observation suggests that mitochondria are a significant source of free radical production during reperfusion. The central hypothesis of this proposal is that during early reperfusion, the sudden reactivation of mitochondrial electron transport results in overwhelming free radical formation and damage, outweighing the benefit of renewed ATP production. The antioxidant systems, impaired during ischemia, may be unable to protect against the uncontrolled reactivation of mitochondrial activity and may contribute to further cell injury. By contrast, reperfusion combined with transient respiratory inhibition could reduce reactive oxygen formation and damage, and allow time for restoration and supplementation of antioxidant defenses. In addition, the concurrent stimulation of glycolytic ATP would partially compensate for the loss of respiratory ATP, would not result-in free radical formation, and could provide ATP where it is most needed, near the sarcolemma. Subsequent washout of the electron transport inhibitors would then restore respiration, but at a stage when endogenous antioxidant defenses are restored and exogenous antioxidant supplementation may occur. The perfused cardiomyocyte model is well-suited for this proposed study because it allows serial measures of oxygen consumption, free radical formation and damage, and direct observation of cells undergoing injury and death during ischemia/reperfusion. The University of Chicago provides an outstanding environment for this research. The Section of Emergency Medicine and the Department of Medicine are committed to supporting the candidate. The advisory committee for the candidate includes experts in energetics, oxygen consumption, free radical formation and defense. Two senior sponsors will mentor the candidate to assure his continued development as an independent researcher.